It was narrated from Maimunah that when the Prophet (ﷺ) prostrated, he would hold his forearms away from his sides, such that if a lamb wanted to pass under his arms, it would be able to do so.
حَدَّثَنَا هِشَامُ بْنُ عَمَّارٍ، حَدَّثَنَا سُفْيَانُ بْنُ عُيَيْنَةَ، عَنْ عُبَيْدِ اللَّهِ بْنِ عَبْدِ اللَّهِ بْنِ الأَصَمِّ، عَنْ عَمِّهِ، يَزِيدَ بْنِ الأَصَمِّ عَنْ مَيْمُونَةَ، أَنَّ النَّبِيَّ ـ صلى الله عليه وسلم ـ كَانَ إِذَا سَجَدَ جَافَى يَدَيْهِ فَلَوْ أَنَّ بَهْمَةً أَرَادَتْ أَنْ تَمُرَّ بَيْنَ يَدَيْهِ لَمَرَّتْ .
IbnMaja
Establishing the Prayer and the Sunnah Regarding Them • Hadith 880